The production of line art or halftone multicolour colloid patterns is important commercially in the field of design, e.g. in the production of colour decorative patterns, colour wiring and circuit diagrams, cartography, colour proofing, the preparation of transparencies for projection in transmission or the preparation of opaque prints for overhead projection in a reflection mode.
Especially in the colour field of the graphic art there is a great need for a simple and fast technique offering "colour proofs" of high quality and reproducibility.
Photographically produced colour proofs are a substitute for multicolour halftone reproductions as are produced by successive printing in registration with the separate standard inks: magenta, yellow, cyan and black on a conventional printing press. A process for the production of colour proofs by preparing a printing plate and running the plate on the press to produce only a few copies as proof of the quality of the halftone separation transparencies used in the exposure is a very expensive procedure and therefore more economic photographic systems have been developed to obtain a similar result by means of which the appearance of a print starting from particular colour separation negatives or positives can be judged by the printer and client.
According to a process known from U.S. Pat. No. 3,642,474 the production of such a proof in the form of superposed coloured colloid patterns, employs a plurality of hydrophilic differently coloured colloid layers, each of which is hardened by irradiation by means of an active species formed during or after an informationwise exposure to active electromagnetic radiation of a photosensitive substance, and comprises the following steps:
(1) transferring a coloured hydrophilic colloid layer containing said photosensitive substance and a hydrophilic colloid which undergoes a reduction in solubility in water by said active species, from a relatively hydrophilic temporary support to a less hydrophilic permanent support by pressing the latter in the presence of an aqueous liquid against said colloid layer, and removing the temporary support, thus leaving the said layer on the permanent support; PA1 (2) exposing the transferred colloid layer in substantially dry state to active electromagnetic radiation, which is modulated according to the information to be recorded, PA1 (3) hardening developing the exposed layer by means of an aqueous liquid followed by a wash-off processing resulting in a coloured relief pattern, and repeating the steps (1), (2) and (3) with said other differently coloured hydrophilic colloid layers to form superposed coloured colloid patterns on a single permanent support.
In the above process for the production of colour proofs, the imagewise exposure proceeds in a vacuum frame with the particular separation image of the multicolour pattern to be reproduced in contact with the coloured hydrophilic colloid layer on the permanent support.
The several contact exposures proceed in registration on the same permanent support which received the unexposed coloured imagewise hardenable hydrophilic colloid layers by transfer one-by-one from a temporary support, each transfer and exposure being followed by imagewise hardening, wash-off processing and a drying step so as to form an additional selected colour pattern on top of an already formed one.
The above sequence of processing steps as defined in U.S. Pat. No. 3,642,474 is not suited for rapid access imaging in that each exposure step is interrupted by wet processing steps and a drying step so that the production of a 4-colour proof takes at least 22 minutes.
A more rapid multicolour image production is possible by a change in the sequence of handling so that each contact exposure of a selected monochrome layer takes place while that layer is on its own temporary support and is not delayed by processing steps. The exposed layers are transferred in registration and processed after being individually transferred to the same permanent support. Such procedure although giving a quicker access to the final multicolour image has the important disadvantage that by transferring the exposed layer onto the permanent support a reverse reading image is obtained thereon when as is necessary for obtaining optimal image resolution and sharpness the contact exposure of the photosensitive hardening developable coloured hydrophilic colloid layer on the temporary support has been carried out with the hardening developable layer in contact with the image side of the selected colour separation negative or positive. Indeed, since in lithographic printing by the offset process the plates must be straight or right reading, the line and/or halftone colour separation images used for the desired contact-exposure of the plates are reverse or wrong reading as seen from the image-bearing side.